When a malfunction is detected in a power supply circuit for a load such discharge tube etc., it is necessary to stop the supply of power so as to protect the load and power supply circuit. Conventional power supply circuits equipped with such a protection function are, for example, disclosed in Japanese patent publication No.04-070869 (1992) and Japanese patent application laid-open No.10-052068 (1998).
FIG. 1 shows a first example of the conventional power supply circuits, which is disclosed in Japanese patent publication No.04-070869 (1992). This power supply circuit is composed of a load voltage detection means with resistors 112 and 113, a first reference voltage generation means with a resistor 126 and a capacitor 127, a second reference voltage generation means with resistors 123, 124 and a capacitor 115, an IC section 100 that generates power-supply voltage based on the two reference voltages and the load voltage detection value, a power amplification section with resistors 104, 107, a transistor 105 and a main transistor 106, a transformer 108 that is driven by the main transistor 106, a rectification diode 109 connected to the secondary coil of the transformer 108, and a capacitor 110 to smoothing the output of the diode 109.
The IC section 100 is composed of a comparator 101 to which the first reference voltage is input, an output transistor 102 which is connected to the comparator 101 and drives a transistor 105, an error amplifier 114 to which the load voltage detection value is input, a comparator 121 connected to the error amplifier 114, a transistor 116 connected to the comparator 121, a resistor 125 as a load of the transistor 116, transistors 117, 122 that drives the resistors 124, 125, a comparator 118 to which the output voltage of the transistor 117 and reference voltage Vr are input, a latch circuit 119 connected to the comparator 118, a transistor 120 which is driven by the latch circuit 119 and controls the input stage of the output transistor 102, and an oscillator 128 which outputs oscillation frequency to the comparator 101.
In FIG. 1, the output transistor 102 is actuated by out put voltage VD of the comparator 101. With the start of the output transistor 101, the transistors 105, 106 are actuated. The main transistor 106 drives the primary coil of the transformer 108 by frequency output from the comparator 101, thereby generating boosted AC voltage at the secondary coil. The output of the transformer 108 is rectified by the diode 109, then smoothed by the capacitor 110, output as DC voltage from the output terminal 111. The terminal voltage (Vout) of the capacitor 110 is taken out through a voltage divider circuit with the resistors 112 and 113, input to one terminal of the error amplifier 114, compared with the reference voltage Verr. Output voltage VB according to this comparison result is input, as one input voltage, to the comparator 101. The comparator 101 compares output VA of the oscillator 126, output voltage VB of the error amplifier 114 and terminal voltage VC of the resistor 124. The transistor 102 is driven by a pulse cycle according to the comparison result, and finally the turn-on state of the transformer 108 is controlled. According to the turn-on control, the secondary-coil output voltage of the transformer 108 varies.
On the other hand, according to the output voltage of the comparator 121, the transistors 116, 122 and 117 are driven and terminal voltage VC of the resistor 124 varies. The variation of terminal voltage VC is compared with the reference voltage Vr by the comparator 118. When VC&gt;Vr, the comparator 118 outputs a voltage. The output of the comparator is latched by the latch circuit 119, and the transistor 120 is turned on by the latch output. By the turn-on of the transistor 120, the base voltage of the output transistor 102 comes to zero level. Since the transistors 102, 105 and 106 are turned off, the transformer 108 outputs no voltage.
FIG. 2 shows the operation at some parts of the power supply circuit in FIG. 1. At the comparator 101, of output voltage VB of the error amplifier 114 and output voltage VC of the resistor 124, either one with lower voltage is compared with VA. At the start of operation, output voltage Vout is zero and output voltage VE divided by the resistors 112, 113 is also zero. The divided voltage VE is compared with reference voltage Verr by the error amplifier 114. When the divided voltage VE is zero, output voltage VB of the error amplifier 114 scales out to the fullest extent in the positive direction. Thus, the comparator 101 compares output voltages VA and VC, and, after the start of operation, the potential (terminal voltage VC) of the capacitor 115 increases gradually from zero while being charged. Therefore, the pulse width of output voltage VD of the comparator 101 also increases gradually. Thereby, the soft starting begins. During this time, since output VB of the error amplifier 114 is above reference voltage Vi, the output of the comparator 121 becomes high level and the transistor 116 turns on. Thus, the transistors 122, 117 give a charge to the capacitor 115. Before terminal voltage VC of the capacitor 115 exceeds reference voltage Verr, output voltage Vout of the output terminal 111 reaches a given value. The input of the error amplifier 114 is balanced to be VE .apprxeq.Verr, and output voltage VB becomes less than reference voltage Vi, thereby the transistor 116 is shut off. Here, terminal voltage VC of the capacitor 115 is stabilized by a voltage determined by the resistors 123, 124.
When the short-circuiting between the output terminals 111 occurs during the stable operation of the power supply circuit, output voltage Vout becomes zero. Therefore, output voltage VB of the error amplifier 114 shifts to the positive side, and exceeds reference voltage Vi. As a result, the transistor 116 turns on, and the capacitor 115 is charged by the transistor 117. When the terminal voltage of the capacitor 115 exceeds reference voltage Verr, the output voltage of the comparator 118 turns into high level, and is latched by the latch circuit 119. Thus, the transistor 120 turns on and the output transistor 102 turns off.
The power supply circuit in FIG. 1 has a characteristic that circuits for both soft-starting and short-circuit protection can be configured by using one capacitor 115. Also, in the process of operating the protection circuit, by providing a delay time before operating the protection circuit, the protection circuit can be prevented from malfunctioning due to a load short-circuiting in short time or a noise.
FIG. 3 shows a second example of the conventional power supply circuits. This power supply circuit is used as an inverter device to turn on a discharge lamp and is disclosed in Japanese patent application laid-open No.02-065100 (1990).
The inverter device is composed of a diode bridge 201 for power supply, a capacitor 202 for the smoothing connected to the output of 201, a resistor 203 connected to a detection power-supply line PL1, a switch 204 connected to the output side of the resistor 203, a resistor 205 connected to the switch 204, a capacitor 206 connected to the output side of the resistor 203, a diode 207 connected to the output end of the detection power-supply line PL1, resistors 208, 209 connected between the anode of the diode 207 and the earth line, a transistor 210 to the base of which a voltage divided by the resistors 208, 209 is input, a resistor 211 connected between the anode of the diode 207 and the collector of the transistor 210, a resistor 213 between the collector of the transistor 210 and the earth line, a transistor 214 to which a voltage at the connection point between resistors 211 and 213, resistors 210, 212 connected between the collector of the transistor 214 and the detection power-supply line PL1, a transistor 215 whose base is connected to the connection point of the resistors 210, 212, resistors 216, 217 and 219 connected between the collector of the transistor 215 and the earth line, a transistor 218 to the base of which the output voltage of the resistor 215 is input and whose emitter is connected to the connection point of the resistors 217 and 219, a resistor 220, a diode 221 and a resistor 222 that are connected between the emitter of the transistor 218 and the output of the diode bridge 201, a capacitor 223, a transistor 227 and a diode 228 that are inserted in series between the output of the diode bride 201 and the earth line, a diode 229 connected between the collector of the transistor 227 and the cathode of the diode 228, a ballast transformer 232 with a third coil N3 connected between the detection power-supply line PL1 and the earth line, a diode 230 connected to one end of the coil N3, a capacitor 231 connected between another end of the coil N3 and the diode 230, a capacitor 223 connected between the collector of the transistor 227 and the output of the diode bridge 201, a coil 224 that is provided with an intermediate tap and connected between the output of the diode bridge 201 and the first coil N1 of the ballast transformer 232, a capacitor 225 and a coil 226 that are connected in series between the base of the transistor 227 and one end of the second coil N2, discharge lamp 233 (TLP), as a load, that is connected between the output of the diode bridge 201 and another end of the first coil N1, and a capacitor 234 connected between the filaments of the discharge lamp 233.
When the switch 204, which is a dimmer switch, turns on, the resistor 205 connected in series to the switch 204 is connected in parallel to the resistor 208, and then the base voltage of the transistor 210 increases abruptly, turning on the transistor 210. Thus, the normal operation of dimmer can be conducted.
FIG. 4 shows the change of terminal voltage of the capacitor 206 in the power supply circuit in FIG. 3. The operation of the power supply circuit in FIG. 3 is explained below referring to FIG. 4.
The alternating-current power supply AC is rectified and smoothed by the diode bridge 201 and the capacitor 202. When power is applied through the coil 224 to the first coil N1 of the ballast transformer 232, the feedback is conducted from the second coil N2 of the ballast transformer 232 through the coil 226 and the capacitor 225 to the base of the transistor 227, thereby the transistor 227 oscillates. Thus, the self-excited oscillation of the transistor 227 starts, the first coil N1 is continuously fed with a given cycle of current, a radio-frequency voltage is continuously output between both ends of (first coil N1+coil 224), thereby pre-heating current is supplied to the discharge lamp 233. The radio-frequency voltage generated is taken out from the control coil N3 of the ballast transformer 232. Then, a voltage rectified and smoothed by the diode 230 and the capacitor is taken out through the detection power-supply line PL1, then supplied to the control circuit composed of the transistors 210, 214, 215 and 218.
At the beginning of power supplying, since the voltage to charge the capacitor 206 does not occur, the transistor 214 is turned off. Therefore, the transistors 215, 218 are turned off. Also, the base feedback resistance of the transistor 227 is a high resistivity by the serial resistors 219, 220, and the transistor 227 is in the operation dependent on the resistor 222. Thus, the discharge lamp 233 comes into the dimming state, where the pre-heating of the discharge lamp 233 is conducted.
As shown in FIG. 4, the capacitor 206 is gradually charged with time through the resistor 203. When the terminal voltage increases more than a predetermined value at time t1, the transistor 214 turns on. In response to this, the transistors 215, 218 also turn on, both ends of the resistor 219 are short-circuited, the base feedback resistance of the transistor 227 is as low as nearly only the resistor 220, thus coming into the full-drive state. Therefore, a high voltage is generated at the coil N1, thereby the discharge lamp 233 turns on.
Here, when the discharge lamp 233 does not turn on due to the deterioration of lighting performance, the load impedance increases offering the light-load state and the output voltage (terminal voltage of the discharge lamp 233) increases gradually. In this state, the voltage at both ends of the capacitor 206, as shown in FIG. 4, continues to increase even after passing time t2. Then, after a given time (time t3) determined by a time constant for the resistor 203, the capacitor 206, the resistor 208 and the resistor 209, the transistor 210 turns on and all the transistors 214, 215 and 218 are cut off. Thereby, the power supply circuit turns into the dimmer mode, where the inverter output voltage is lowered and the components of the circuit are protected from high voltage. Meanwhile, when the discharge lamp 233 turns on normally, the voltage at both ends of the capacitor 206 does not increase to such high level, therefore the transistor 210 does not turn on and the inverter circuit continues to light the discharge lamp 233 at the rated voltage.
FIG. 5 shows the composition of a conventional power supply circuit using a piezoelectric transformer. This power supply circuit is used as an inverter that drives cold-cathode tube as a load. The details of this circuit is disclosed in Japanese patent application laid-open No.10-052068 (1998). In inverters using a piezoelectric transformer, a protection circuit is provided so as to prevent the deterioration of piezoelectric transformer's characteristic and to reduce the heat generation of circuit component in the opening of load.
The power supply circuit is composed of a piezoelectric transformer 301 with a load 302 connected, a frequency control circuit 303 connected to the load 302, a booster circuit 304 connected to the piezoelectric transformer 301, an overvoltage protection circuit 310 connected to the load 302 and the frequency control circuit 303, and a drive voltage control circuit 311 connected to the frequency control circuit 303 and the booster circuit 304.
FIG. 6 shows the output characteristic of the piezoelectric transformer 301. The piezoelectric transformer 301 is composed of a primary side electrode and a secondary side electrode formed on a plate-like piezoelectric ceramics. By applying AC voltage with resonance frequency to the primary side electrode, output voltage is generated at the secondary side electrode by piezoelectric effect. The piezoelectric transformer 301 has high output impedance and its operation depends on load impedance. Therefore, as shown in FIG. 6, when the load impedance is high, the boost ratio increases and thus high voltage is output. The piezoelectric transformer 301 with such structure and characteristic has advantages that can offer a miniaturized and thinned body, compared with electromagnetic transformers. Thus, the use of a power supply for backlight LCD etc. attracts attentions.
The frequency control circuit 303 is composed of a current-voltage conversion circuit 312 to convert the current through the load 302 to a voltage value, a rectification circuit 313 to rectify the output of the current-voltage conversion circuit 312, a comparator 314 to compare the output of the rectification circuit 313 with reference voltage Vref, an integrator 315 to conduct the integration based on the output voltage VP1 of the overvoltage protection circuit 310, the output voltage of the comparator 314 and the output voltage of a comparator 316, the comparator 316 to compare the output voltage of the integrator 315 and reference voltage Vmin, a VCO (voltage-controlled oscillator) 317 to output control voltage Vr and Vvco based on the output voltage Vint of the integrator 315
The booster circuit 304 is composed of a first automatic transformer 305, a second automatic transformer 306, a first switching transformer 307, a second switching transformer 308, and a double phase drive circuit 309 to drive the first and second switching transformers 307, 308. The double phase drive circuit 309 drives the first and second switching transformers 307, 308 based on the control voltage Vvco output from the VCO 317.
The overvoltage protection circuit 310 is composed of a voltage division circuit 318 to divide the output voltage of the piezoelectric transformer 301, a rectification circuit 319 to rectify the output voltage of the voltage division circuit 318, and a comparison block 320 to compare the output voltage of the rectification circuit 319 with comparison voltage Vmax.
The operation of the booster circuit 304 is explained below. By reverse-phase clock output from the double drive circuit 309, the first switching transistor 307 and the second switching transistor 308 are turned on alternately. Thereby, power is supplied form the power supply VDD to the primary side of the first and second automatic transformers 305, 306, charged as current energy. When the first and second switching transistors 307, 308 are turned off alternately, the first and second automatic transformers 305, 306 discharge the charged energy. This charged energy is converted into voltage resonance waveform by the equivalent input capacitance of the piezoelectric transformer 301 and the load 302 and the summed inductance of the primary-side inductance and the secondary-side inductance of the automatic transformer, then applied to the primary side electrode of the piezoelectric transformer 301.
The operation of the frequency control circuit 303 is explained below. Load current Io of the load 302 is converted into a voltage value by the current-voltage conversion circuit 312, then converted into DC voltage by the rectification circuit 313. This DC voltage is compared with reference voltage Vref by the comparator 314. When the DC voltage is low, the comparator 314 outputs such signal that the output of the integrator 315 is increased at a constant rate, to the integrator 315 The signal output from the integrator 315 is input to the VCO 317. The VCO 317 outputs a frequency pulse inversely proportional to a voltage value input. By half of this frequency, the transistors 307, 308 are driven, and thereby the piezoelectric transformer 301 is driven. Accordingly, when load current Io is less than a predetermined value, the drive frequency of the piezoelectric transformer 301 continues to lower.
As shown in FIG. 6, since the piezoelectric transformer 301 is set so that it has a drive frequency f lower than its start point of frequency f1, the boost ratio of the piezoelectric transformer 301 increases as drive frequency f comes close to resonance frequency fr and load current Io increases with time. When drive frequency f continues to lower until a voltage input to the comparator 314 is higher than reference voltage Vref at drive frequency f0 as shown in FIG. 6, the comparator 314 generates output signal so that the output of the integrator 315 continues to hold a latest output value. As a result, the output frequency of the VCO 317 becomes constant, the piezoelectric transformer 301 is driven at constant frequency f0 and load current Io becomes constant.
After the piezoelectric transformer 301 starts driving at constant frequency, when load current Io varies due to a variation in impedance of the load 302 etc. thereby input voltage of the comparator 314 becomes lower than reference voltage Vref, the drive voltage of the piezoelectric transformer 301 begins to lower again. In the stage that the input voltage of the comparator 314 is not above reference voltage Vref, when the drive frequency of the piezoelectric transformer 301 continues to lower, the drive frequency reaches f2 as shown in FIG. 6. Since at f2, the output voltage Vint of the integrator 315, which corresponds to the input of the comparator 316, is higher than reference voltage Vmin, the comparator 316 outputs reset signal Vr to the integrator 315 When the integrator 315 is reset, output voltage Vs becomes minimum. As a result, the output of the VCO 317 comes to the highest frequency and the piezoelectric transformer 301 is driven at drive frequency f1. From this stage, the drive frequency of the piezoelectric transformer 301 starts to lower again. In this operation, when a frequency where the input voltage of the comparator 314 becomes higher than reference voltage Vref is detected, the output voltage of the integrator 315 is locked and the output frequency of the VCO 317 becomes constant.
The double-phase drive circuit 309 generates output voltages Vg1, Vg2 with different phases. The double-phase drive circuit 309 repeats the inversion between output Vg1 and Vg2 each time Vvco is input from the frequency control circuit 303. The drain voltage Vd1 of the first switching transistor 307 is input to the drive-voltage control circuit 311. When the value of drain voltage Vd1 is above a predetermined value, the input voltage from VDD is output, in time-division manner, to the first automatic transformer 305 and the second automatic transformer 306. The frequency of time-division outputting is determined by Vvco from the VCO 317. Therefore, even when the power source VDD varies, it can keep a high power transform efficiency, as an inverter.
The overvoltage protection circuit 310 is provided to prevent the piezoelectric transformer 301 from being self-destroyed by the over-vibration when the booster ratio becomes high due to high load impedance. To the voltage divider circuit 318, piezoelectric transformer output voltage Vo output from the secondary-side electrode of the piezoelectric transformer 301 is applied. The output voltage of the voltage divider circuit 318 is converted into DC voltage Vr by the rectification circuit 319, then input to the comparison block 320. The comparison block 320 compares the output voltage of the rectification circuit 319 with reference voltage Vmax, and when Vr&gt;Vmax, it outputs two output signals Vp1 (signal to reset the integrator 315) and Vp2 (signal to change the upper limit value of output frequency from the VCO 317). Vp1 is signal that is output only when input voltage is higher than reference voltage Vmax, and Vp2 is signal that is continuously output for a certain time (time required until the output of the integrator 315 changes from the minimum voltage to the maximum voltage) when input voltage is higher than reference voltage Vmax.
FIG. 7 shows the operation of the power supply circuit in FIG. 5. In FIG. 7, the relationships among drive frequency of the piezoelectric transformer 301, piezoelectric transformer output voltage Vo and output signals Vp1, Vp2. The output of the integrator 315 comes to the minimum voltage when output signal Vp1 is input. Therefore, the division ratio of voltage is set so that output voltage Vo, at a voltage higher than which the piezoelectric transformer 301 may incur deterioration in characteristic, is equal to reference voltage Vmax (Vo=Vmax) after passing the rectification circuit 319.
When the load 302 is a cold-cathode tube used for backlight of LCD for notebook computer, the rated power of the piezoelectric transformer 301 is generally about 4 W. In this piezoelectric transformer 301, by setting the maximum of output voltage Vo about 1500 to 2000 V, the deterioration in characteristic can be avoided, and the voltage can exceed the lighting start voltage of the cold-cathode tube (load 302). In FIG. 6, frequency at which output voltage of the rectification circuit 319 is higher than reference voltage Vmax is f3. The upper limit of drive frequency of the piezoelectric transformer 301 is normally set at frequency f1 in FIG. 6, but it is switched to f4, maximum frequency in the period that output voltage Vp2 is input to the VCO 317.
When output voltage Vo of the piezoelectric transformer 301 is lower than a predetermined value and load current Io is smaller than a predetermined value, the frequency of the piezoelectric transformer 301 is scanned between frequencies f1 and f2 in FIG. 6. Also, when output voltage Vo of the piezoelectric transformer 301 is higher than a predetermined value and load current Io is smaller than a predetermined value, the frequency of the piezoelectric transformer 301 is scanned between frequencies f4 and f3 in FIG. 6. Here, a case that output voltage Vo of the piezoelectric transformer 301 is higher than a predetermined value and load current Io is smaller than a predetermined value is, for example, when the load is opened due to the breaking of connection line. In the opening of load, since the load impedance is high, output voltage Vo of the piezoelectric transformer 301 increases, and the load current is at zero since the load is disconnected. In this situation, since the load is not likely to be connected and the load current Io is short of a predetermined value, the drive frequency of the piezoelectric transformer 301 is continuously scanned.
The resonance waveform to drive the piezoelectric transformer 301 is controlled so that it is half-wave type sine wave by setting the inductance of the automatic transformers 305, 306 to optimize the zero-switching at frequency f0 in FIG. 6. Thereby, the power transform efficiency as an inverter can be optimized.
In the composition in FIG. 6, when output voltage Vo of the piezoelectric transformer 301 is higher than a predetermined value and load current Io is smaller than a predetermined value (i.e., when the load is opened), the frequency of the piezoelectric transformer 301 is continuously scanned for a long time or infinite time. So, in FIG. 6, the upper-limit of scanning frequency is switched from f1 to f4. By this switching, the break of zero-switching of resonance waveform at the automatic transformers 305, 306 can be reduced, and thereby the heat generation of component can be reduced.
Meanwhile, in an inverter using a cold-cathode tube as a load, when a malfunction is detected after turning on the power supply VDD, there are two incompatible requirements in a time period until when the overvoltage protection circuit starts to operate. The first requirement is to shut off the output after continuing the inverter output for at least 5 to 6 seconds after turning on the power supply. Also, the second requirement is to stop the circuit operation to drive the piezoelectric transformer 301 immediately, e.g. about 0.1 second, after turning on the power supply to shut off the output. The reasons are explained below.
The reason for the first reason is as follows. When the atmosphere used is at low temperature, a time of about one second may be required from the start of voltage application to the load (cold-cathode tube) to the lighting. In such environment, since the impedance of cold-cathode tube before the lighting is high, a time longer than usual is needed until reducing to such an impedance value that the lighting can be started after starting the voltage application. Therefore, to guarantee the lighting of cold-cathode tube even at low temperature, it is necessary to conduct the shut-off operation of the protection circuit after the non-lighting state of the cold-cathode tube continues for more than 5 to 6 seconds. When the setting is that the operation is stopped after 0.5 second since turning on the inverter power supply under the environment that the cold-cathode tube turns on after one second since turning on the inverter power supply, the shut-off circuit must start to operate despite the condition that the cold-cathode tube turns on after one second since turning on the inverter power supply. Thus, the circuit to drive the piezoelectric transformer 301 is stopped and therefore the cold-cathode tube is not turned on. Meanwhile, recently, to improve the safety, it is desired that the inverter circuit is stopped completely, instead of changing the operation mode as described in Japanese patent application laid-open No.02-065100 (1990), when the cold-cathode tube does not turn on.
The reason for the second requirement is as follows. There is a case that in a malfunction such as an open/short test, the component of booster circuit may emit smoke before the fusing of fuse. This is because when a coil or switching transistor composing the booster circuit is open-circuited or short-circuited and the switching operation of switching transistor is therefore stopped, current larger than usual flows into the booster circuit, thereby causing the emission of smoke. In general, fuses are selected so that the value of current flown at the normal state is less than 80% of fuse rated value. Therefore, even when current larger than usual flows in a moment of time, the fuse may not be fused. In such case, before the fusing of fuse, the component of booster circuit may emit smoke. However, the occurrence of such a malfunction cannot be accepted from the viewpoint of safety. Namely, when a malfunction is detected, it is necessary that the circuit to drive the piezoelectric transformer 301 is stopped within a time (e.g. 0.1 second).
The first and second requirements mentioned above are opposite to each other about the time until the start of shut-off operation. However, both are indispensable requirements in operating a power supply circuit using a piezoelectric transformer.
The conventional power supply circuits have some problems described below.
(1) For excessively-small load current, they cannot determine whether the piezoelectric transformer is need to continue operating or needed to stop operating immediately. For example, in case of Japanese patent application laid-open No.10-052068 (1998), when one of the first and second switching transistors (307, 308) stops operating while lighting the cold-cathode tube using the piezoelectric transformer drive means, the piezoelectric transformer continues to operate, despite the state that the output voltage is short of a predetermined value, by another switching transistor being operated.
FIG. 8 shows waveforms at some parts in the case that one of the switching transistors stops operating. (a) in FIG. 8 shows the drive frequency of the piezoelectric transformer 301. Here, since load current Io is short of a predetermined value, it is scanned between frequencies f1 and f2 in FIG. 6. (b) in FIG. 8 shows load current Io. The load current has a large value instantaneously when the drive frequency passes through around the resonance frequency fr of the piezoelectric transformer 301. (c) in FIG. 8 shows output voltage Vo of the piezoelectric transformer 301. Since the impedance of cold-cathode tube is reduced to a value to allow the lighting, the effective value is slightly lower than the output voltage in the normal lighting.
However, for the power supply circuit in Japanese patent application laid-open No.10-052068 (1998) which has such a composition that a malfunction is detected monitoring only one state of outputting, it is impossible to distinguish between the normal lighting and the malfunction of booster circuit while monitoring the output voltage Vo since there is not a significant difference between the two states. Therefore, since the piezoelectric transformer 301 must be operated continuously even in the malfunction of booster circuit, the component of the booster circuit may be broken.
The effective value of load current Io in the malfunction of booster circuit is near to zero as shown by (b) in FIG. 8 and the load current is also at zero when the load is opened. Therefore, these can be distinguished from the case of normal lighting. However, it is impossible to distinguish between a case that the piezoelectric transformer 301 is needed to stop operating immediately (e.g., 0.1 second after) as in case of malfunction of booster circuit and a case that it is needed to continue operating for at least several seconds as in the opening of load.
(2) They cannot change the time constant of timer circuit until stopping the drive circuit or switching the operation mode. The power supply circuits (inverters) mentioned above are composed so that when detecting a malfunction, they stop the drive circuit or switch the operation mode using the single time constant regardless of the cause of the malfunction. Thus, they cannot conduct proper control according to the state of operation.
(3) They do not satisfy the requirement to stop the output for safety or the requirement to provide a power saving function in case of stopping the output. The circuit in Japanese patent application laid-open No.02-065100 (1990) can change the operation mode but cannot shut off the output. In recent years, with the enhanced requirement to safety, it is required to stop outputting when detecting the non-lighting of cold-cathode tube or the opening of load. On the other hand, the circuit in Japanese patent application laid-open No.10-152068 (1998) stops outputting but does not stop the operation of OSC, comparators, transistors etc. When the outputting is shut off and the circuit is not operated normally, it is desired that the power supply to the electronic components not operated be shut off from the viewpoint of power saving. However, this circuit is not adapted to such a requirement.